Arsenic (As) is an extremely toxic carcinogenic metalloid pollutant that adversely affects the health of millions of people worldwide. Inorganic forms of arsenic, such as arsenate (AsO43−) and arsenite (AsO33−), are more toxic than organic forms of arsenic and cause cancer. Arsenic poisoning can occur via ingestion of contaminated drinking water and food. Industrial pollution and agricultural practices including the use of arsenic in pesticides, herbicides, fertilizers, wood preservatives, mining, and irrigation with contaminated groundwater have significantly arsenic levels in agricultural soil. The arsenic contaminated soil, sediment, and water supplies are major sources of contamination in the food chain. There is global concern regarding arsenic contamination in drinking water and soil, particularly on the Indian subcontinent where more than 450 million people are at risk for arsenic poisoning.
There are many different ways arsenic can enter the food chain. Plants grown on arsenic contaminated soil can accumulate high levels of arsenic in roots, shoots, and grain. Arsenic uptake by plants may play an important role in the introduction of arsenic into the food chain, for example, by the direct ingestion of arsenic contaminated grain. In addition, arsenic contaminated straw that is used as cattle feed may have adverse health effects on cattle and may result in an increased arsenic exposure in humans via a plant-animal-human pathway. There is, therefore, concern regarding the accumulation of arsenic in meat and dairy products as well as in agricultural crops and vegetables.
In addition, arsenic is phytotoxic and causes significant loss in crop yields. Arsenate is a phosphate analog and competes with phosphate for uptake in plants causing the inhibition of phosphate and other nutrients. Thus, arsenic contamination is an agricultural concern.
Arsenic is present in the environment in different forms. The arsenate oxyanions, HAsO42− and H2AsO4−, are the most prevalent forms of arsenic in surface soil, water, and within cells, and these oxyanions contain arsenic in the pentavalent state [As(V)]. Arsenite, which at neutral pH contains arsenic in the trivalent oxidation state [As(III)] and likely as the acid HAsO32−, is highly reactive and readily forms As(III)-thiol complexes. Plants use arsenate reductases to detoxify arsenic by reducing As(V) to As(III), which is subsequently detoxified via forming complexes with thiol-reactive peptides such as γ-glutamylcysteine (γ-EC), glutathione (GSH) and phytochelatins (PCs). It is suggested that these AsIII-thiol complexes are then sequestered into vacuoles by glutathione-conjugating pumps. It is further believed that plants trap arsenite in below ground tissues in order to prevent access to aboveground reproductive tissues to prevent possible mutagenic consequences.
The structure and function of arsenate reductases, particularly bacterial arsenate reductases, has been studies extensively. Arsenate reductases reduce arsenate [As(V)] to arsenite [As(III)]. Arsenate reductases include a P-loop with a characteristic CX5R sequence motif flanked by a beta-strand and an alpha-helix. The arsenate substrate undergoes a nucleophilic attack by the thiol of the cysteine in the P-loop. A hydroxyl then leaves the arsenic, leading to a covalent Cys-HAsO intermediate. The nucleophilic displacement is followed by an intramolecular disulfide bond cascade with two other redox-active cysteines in which arsenite is released and an intramolecular disulfide bond is formed. After completion of the reaction, the arsenate reductase is regenerated by thioredoxin that reduces the disulfide bond formed during the reaction. Arsenate reductases are very efficient at detoxifying arsenate ions.
There is a strong need to reduce the arsenic uptake in food crops and the subsequent introduction of arsenic into the food chain. There further is a need to develop crops that are resistant to arsenic.